How do I use high order unicode characters in java?

You need to specify it as a surrogate pair - two UTF-16 code units.

For example, if you copy and paste the character into my Unicode explorer you can see that U+1F174 is represented in UTF-16 code units as U+D83C U+DD74. (You can work this out manually, of course.) So you could write it in a Java string literal as:

String text = "\uD83C\uDD74";

Other options include:

String text = new StringBuilder().appendCodePoint(0x1f174).toString();
String text = new String(new int[] { 0x1f174 }, 0, 1);
char[] chars = Character.toChars(0x1f174);